Band-pass filter



May 5, 1942. N. MgRUsr ETAL -BAND PASS FILTER Filed April 30, 1941 uuuu n 1111 11 -g l. 000 V I I I I f INVENTORS IVOEL M. RUST, JOSEPH D BMILSFORD AND ERNES T El GOODE/VL/GH y' TTORNEY Patented May 5, 1942 BAND-Pass mma y Nol Meyer` Rust, Ghelmaford, Joseph Douglas.

Brailsford, London, and Ernest Frederick Goodenough, Springfield, Chelmsford, England, assignora to Radio Corporation of America, a

corporation of Delaware appuennnn apra 3o, 1941, vsenin Np. v:91,2205 In Great Britain October 5, 1939 7 claims. (cl. 1re-171i i This invention relates tu bend-pass miens and f has for its object to provide'improved filters which are comparatively flexible in' design and will facilitate the production of wide band ampliiiers of high gain and large signal-to-noise ratio, or narrow band amplifiers of great selectivity and with a flat topped response curveV such f as will give a good signal-to-noise ratio. Though not limited to its application thereto the primary applications 'of the invention are to television amplifiers, and to intermediate frequency ampliners for broadcast and like receivers.

The present invention may be regarded as a development or variation of the invention contained in our co-pending application Serial No. 359,182, led October l, 1940. According to this said invention a band-pass filter effect is obtained by superimposing upon the frequency-impedance characteristics of one network, the frequencyimpedanceI characteristic of at least one further network which is coupled to the first in such manner that the impedance elements in the second network are, in. effect, reversed. Inother words, there are employed, in accordance with the. invention contained in the specification of the above application, two networks coupled to- -gether in such manner that the second network acts as though it were directly included in and tive and parallel resistance is negative the said two tuned circuits being connected together by means of a quarter waveline 'and the values of the resistances in each case being equal to that oi' the characteristic impedance oi the said line.

The invention is illustrated in and explained4 j in connection-with the drawing accompanying the specification, in which Fig.'1vrepresents a circuit which will serve to explain the principles underlying the invention, Figs. 2 and 3 are modifications of an embodiment of the invention, Fig. 4 is a practical circuit arrangement embodying the invention,l and Fig. 5 represents the electrical equivalent of the circuit shown in F18. 4.

In orderthat the invention may understood there will first be given a brief and simplified description of the phenomena upon which the invention is based.

Consider first a circuit as shown in Figure 1 which is an explanatory diagrammatic figure.

' In this circuit a quarter `wave line (conventionas part of the first network but with its im-` Y pedance elements reversed. The expression reversed as employed in this connection is used to mean inversion and sign reversal of an impedance as set out in the said speciiication. In the various arrangements described in the copending 'application referred to negative impedances o f a value comparable to positive impedances andvarying in the same manner as the freequal and opposite. In

ally indicated by the dimension' 7\/4, i bein the wave length) is terminated by impedance ne works which are similar as regards reactive properties but other words, the equivalent shunt resistance is positive in` one impedance network and negative in the other. Each, moreover, is numerically equal to the characteristic impedance Z0 of the line.

In accordance with the well known properties of a quarter wave line the impedance looking v into the line with the near end termination disquency is varied,V are used' to produce an impedance which is substantially constant over a band.

of frequenciesand is'of a higher value than the original positive impedance to be operated upon.

In other words, negative resistance effects are used to compensate positive resistance effects, and negative` reactance effects to compensate positive reactance effects. 'In these arrangements both negative `effects are produced by valve circuits The present invention differs from the previcuit at the far end over a frequency bandv near resonance.

ous invention above referred to in that, although pure negative resistance produced by avalve arrangement is still usedto cancel positive resistance. the cancellation of reactive eiects is produced by a quarter wave line action. The present invention offers the advantage of being` rather more practical and economical for narrow band working, e. g. for handling ultra short wave television carriers and sidebands.

According toA the present invention a band pass filter comprises at least two tuned circuits including similar reactive elements and in one of which the eifective parallel resistance isposiconnected is'Zu at the in-tune frequency, but at frequencies ofi-tune, this impedance behaves as the inverseof that terminating the far end of the line. Thus-` a parallel resonance circuit will appear as a series resonant circuit of approximately the same Qas that of the parallel resonant circuit and its reactance will change in the opposite direction and at approximately the same rate over a small band of frequency near resonance.

The change in reactance (or rather susceptance) of a circuit shunting the near end will therefore be cancelled by that of a similar cir- At frequencies further from resonance, the susceptance changes differ` and noV longer cancel completely.v

If the resistive component of one terminating impedance be negative, it will produce a negaytive admittance at the other end of the line,

which lwill cancel the positive admittance of the` circuit shunting' that end, giving (theoretically) zero admittance, i. e. infinite impedance at and near the in-tune frequency. The line admittance will change an increasing amount with increasing departure from this' in-tune frequency an cancellation will no longer be complete. 1

inthe other of which the effective be the better whose `resistive components are Measurements taken at point A (to earth) would reveal that at the tune frequency ofthe circuits the negative and positive resistance effects would cancel and the impedance would be pensation effect with an increase of impedance would be obtained, therefore, the limitation being imposed firstly by incomplete resistance cancellation, and secondly by incomplete reactance cancellation.

With an arrangement as shown in Figure 2 the reactance swings (with frequency variation) in the two tuned circuits are identical, but the resistance effects are of opposite sign. The combined result is therefore a cancellation of resistance effects and `a summation of the reversed reactance effects thrown over by the quarter wave lines. A parallel tuned circuit of relatively high Q value shunting the junction of the two lines as in Figure 3, gives reactance swings of opposite sign to those thrown over by the quarter wave lines, and, (over a small band of frequencies) produces reactance cancellation, the resistance component of this circuit stabilising the circuit over all.

,Actially the susceptance thrown over by the two lines increases uniformly at first with change of frequency from resonance, then more slowly and finally decreases. If the initial susceptance change ofthe center circuit be madeslightly less than the initial susceptance change oi' the lines, at some frequency off-tune, the susceptance of the center circuit will equal that of the lines and exact cancellation will take place. Thisarrangement will then give a treble humped response curve, viz., one hump at the tune frequency Fo and two at the frequencies Fi and Fi of susceptance cancellation, spaced logarithmicaliy equidistance from Fo. Outside Fi and Fa .the curve drops sharply ,because the susceptance of the middle circuit increases much more rapidly than that of the lines, and compensation is increasingly reduced.

The resistance component remains unchanged A`(expressed as a parallel resistance) since the only resistance that changes is that of the lines,- each of which changes to the same amount and cancel one another out. Between Fi and F: the imvalve has a condenser of 26 mmfds. and the grid load of the output valve one of 16 mmi'ds.-in

both cases these can be realised with valve and wiring stray capacities: The arrangement of cathode impedance in the output valve limits the negative impedance applied by the back coupling to )I 1,000 ohms, or more logically, the negative admittance applied cannot exceed 1/1000 or .001 ohm.

As will be seen from Figure 5, the coupling system between the circuits is equivalent to quarter wave lines. In this'iigure the quarter wave lines are conventionally indicated as M4 and the reactance correction circuit is indicated as RC.

The shunt inductance of the quarter wave line systems are, of course, lumped in with the tuning coils of the tuned circuits, just as the valve and wiring stray capacities are lumped in with the condensers.

What we claim is:

1. A band pass filter circuit comprising at least two tuned circuits including similar reactive elements and in one of which the effective parallel resistance is positive and in the other of which the eii'ective parallel resistance is negative, the said two tuned circuits being connected together by means of a quarter wave line and the values of the resistances in each case being equal to that of the characteristic impedance of the said line.

2. A filter in accordance with claim 1 wherein the two tuned circuits are connected together by two quarter wave lines in series, there being connected across the filter at the junction of the two lines a parallel tuned circuit of' relatively high Q value.

3. A filter circuit comprising a valve having a tuned anode circuit, a parallel tuned circuit having a positive effective parallel resistance connected Ato said anode circuit over one quarter wave line and a further Yparallel tuned circuit having a negative effective parallel resistance connected to said anode circuit over a second quarter wave length line.

4. A filter circuit as claimed in claim 3 wherein thesaid further tuned circuit is connected to the grid of an output valve.

5. A filter circuit as claimed in claim 3 wherein the said further tuned circuit is connected to the grid of an output valve and there is included in the cathode leg of said ,output valve a resistance pedance is substantially constant and is raisedA very considerably by the impedance. f

In practice the required conditions can be obaction of the negative tained with simple circuits. as shown in Figure 4.

In Figure 4 actual values are indicated by way A.

of example. l

The circuit illustrated in Figure 4 is designed to give at 50 megacycles with a band width substantially fiat of i2 megacycles, a stage gain of the order of 5 0 with television pentodes. It will be seen that the anode circuit load of the first of value equal to said positive effective resistance of the first parallel tuned circuit and wherein the anode oi' said output valve is coupled back to the inductance of said further tuned circuit.

s. A nicer circuit as claimed in claim 3 wherein the said further tuned circuit is connected to the grid of an output valve and wherein the capacitances of said' parallel tuned circuits are constituted, at least in parts, by the capacities of the valves and wiring.

' 7. A filter circuit as claimed in claim 3 wherein the said further tuned circuit is connected to the grid o f an output valve and wherein the inductances of the quarter wave line -are constituted bythe inductances oi.' the tuned circuits.

NOEL MEYER RUST. JosEPn DOUGLAS BRAnsEoRD. ERNEs'r FREDERICK GooDENoUGH. 

